Page 254 - Third Book of Reading Lessons

READING LESSONS. 253
tion, until it is compelled by some force to change i state." This constitutes what is denominated the inertia, or inactivity of matter. And it may be ob­ served, that a change never happens in the motion of any body, without an equal and opposite change in the motion of some other body.
CHARLES.-T· here is no di culty of conceiving that a body, as this inkstand, in a state of rest, must al­ ways remain so, if no external force be impressed upon it to give it motion. But I know of no ex­ ample which will lead me to suppose, that a body once put in motion, would of itself continue so.
FATHER.-You will, I think, presently admit the latter part of the assertion as well as the rmer, al­ though it cannot be established by experiment.
EMMA.-l shall be glad to hear how this is.
FATHER.-You will not deny that the ball which you strike om the trap, has no more power either to destroy its motion, or to cause any change in its velocity, than it has to change its shape.
CHARLEs.-Certainly ; nevertheless; in a few sec­ onds a er I have struck the ball with all my rce, it lls to the ground, and then stops.
FATHER.-Do you nd no di erence in the time that is taken up be re it comes to rest, even suppos­ ing your blow the same ?
CHARLES.--Yes ; if I am playing on the grass it rolls to a less distance than when I play on the smooth gravel.
FATHER.-You nd a like di erence when you are playing at marbles, if you play in the gravel cour , or on the even pavement in the arcade.
CHARLEs.-The marbles run so easily on the smooth
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